white paper
Putting Crystals in Their Rightful Place Comparing the performance of quartz crystal and MEMS oscillators in high-reliability applications Scott Sentz, Q-Tech Corporation
Introduction MEMSnet.org, a website that describes itself as a “Clearing complex electromechanical systems with multiple moving House and information portal for MEMS,” defines this elements under the control of integrated microelectronics. technology as follows, “Micro-Electro-Mechanical Systems, The one main criterion of MEMS is that there “are at least or MEMS, is a technology that in its most general form some elements having some sort of mechanical functionality can be defined as miniaturized mechanical and electro- whether or not these elements can move.” mechanical elements (i.e., devices and structures) that are made using the techniques of microfabrication. The critical MEMS technology is now employed in a wide range of physical dimensions of MEMS devices can vary from well sensor and actuator applications. In addition, an emerging below one micron on the lower end of the dimensional use of MEMS technology is as a replacement for traditional spectrum, all the way to several millimeters. Likewise, the crystal oscillators. This article will take a critical look at types of MEMS devices can vary from relatively simple MEMS vs. Crystals to clarify where each has a place in structures having no moving elements, to extremely advanced electronics design.
Evolution of Oscillator Technologies
The piezoelectric crystal resonator (crystal oscillator) was Mexico, “The core technologies used in MEMS oscillators invented by Dr. Walter Cady and patented in the early have been in development since the mid-1960s but have 1920’s. In 1921 Cady designed the first circuit to control only been sufficiently advanced for commercial applications frequencies based on quartz crystal resonator and since 2006. MEMS oscillators are a valid alternative to older, received two fundamental patents on resonators and their more established quartz crystal oscillators, offering better applications to radio in 1923. Subsequently, crystal oscillators resilience against vibration and mechanical shock and rapidly became the standard for controlling frequency reliability with respect to temperature variation.” in radio communications and other early electronic applications. On the face of this statement, one would perhaps conclude that crystal oscillators have lost their place in the hierarchy The rapid development of higher frequency electronics of standards, but this far from reality. The block diagram applications in the latter part of the last century has driven of a Si MEMS device, as shown in Figure 1 at the top of the development of are spectrum of oscillator technologies. the next page, consists of a MEMS resonator, a stabilizing Lower-cost alternatives to crystal oscillators have been phase-locked-loop and additional circuitry to provide for developed to provide for timing in applications where temperature stability. Applications in aerospace technology, frequency accuracy is less critical. Ceramic resonators, for military guidance systems, downhole drilling controls and example are found in applications like VCRs, telephones, others have evolved quickly, producing an entire range of toys. And more recently, MEMS oscillators have emerged as applications that are far more suited to advanced crystal an alternative to crystals. oscillators than the best-performing MEMS devices. While more costly than MEMS, the intrinsic simplicity of the crystal According to a paper presented in 2008 by the Southwest oscillator adds to their reliability. Center of Microsystems Education at the University of New
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MEMS Performance Claims
A search for details about MEMS oscillator performance Si-MEMS oscillator and 20x lower than the “low-jitter” leads to a lot of conflicting information. In preparation for Si-MEMS oscillator. this article, two extensive reports were reviewed: — Crystal oscillators have the lowest power consumption because they have the advantage A Wikipedia article on the subject is the source for the of the fundamental harmonic oscillation of the citation (above) from the Southwest Center of Microsystems oscillating source and a simple circuit structure. Education. While this is a remarkably informative article, — By contrast, Si-MEMS oscillators consume more Wikipedia actually flags it as having some “content written as power because they have more circuitry. advertisement.” I suppose it is fair to assume that many Wiki n Start-up characteristics posts suffer from this. Even so, the article is well worth the time to peruse. l Using an oscillator with fast startup, allows shorter wakeup cycles and longer battery life than MEMS Perhaps a more relevant source of information, as related devices. to high-reliability applications is found is in a white paper n Frequency temperature characteristics published by Epson in September 2014. While this paper is l On first inspection, the Si-MEMS oscillators seem admittedly produced by a company comparing one of its to have better frequency vs. temperature own crystal oscillators to comparable MEMS devices, the characteristics. Closer inspection shows that the results are worthy of consideration. Si-MEMS oscillators have frequency jumps when the division ratio switches to compensate for The paper compared performance in five critical areas: temperature changes. n Phase noise and phase jitter n Frequency stability l Laboratory measurements demonstrate that the l The “standard” Si-MEMS oscillator has frequency crystal oscillator has much better phase noise than jumps well in excess of most wireless radio standards the either of two Si-MEMS oscillators. The “low-jitter” Si-MEMS oscillator has better n Power consumption performance, but the Quartz oscillator is by far l The crystal oscillator has the lowest power more stable. consumption, 1.5 mA, 5x lower than the “standard”
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What’s the Take-away from this?
MEMS oscillators require more complex circuitry to perform volume consumer products including mobile phones, toys, the basic function and to compensate for temperature games, automotive entertainment/navigation systems, variation and other variables (Figure 1). Despite their to name a few. In some aerospace applications, where intrinsically more complex structure, MEMS are typically short-term performance (one and done) is important but considerably less expensive than crystal oscillators. And with long-term stability is not, MEMS might also be considered. all of the other functions being implemented employing emerging MEMS topologies, the adaption of low-cost So where does that leave the classic high-performance MEMS oscillators is clearly going to increase. Moreover, crystal oscillator? The simple answer might be in high- MEMS oscillators can handle shock and vibration pretty well. performance applications where a “service call” is absolutely On the downside, MEMS oscillators typically exhibit a low Q, out of the question! Space applications like low-earth-orbit which results in the poor phase noise and jitter as evidenced (LEO) and geosynchronous orbit satellites is one area. in the referenced white paper. Finally, MEMS device circuity High-temperature applications like down-hole drilling would simply can’t handle space-related radiation exposure. be another. Military and avionics applications like missile guidance and navigation systems where temperature and Taking all of this into account, there are an enormous frequency stability are critical is another. Crystal oscillators number of applications where MEMS oscillators are are also found in high-definition video displays where the preferred solution. The lower cost and acceptable anything but exceptional performance would be plain to performance levels make these devices ideal for high- see.
Conclusion
MEMS technology is making great inroads into oscillator solution. But the world of ultra-high-reliability is and will applications where their lower cost offers an attractive remain the realm of the venerable crystal oscillator.
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